1. Field of the Invention
This invention pertains to the field of Head-Up Displays (“HUDs”) that provide images in the field of view of a person viewing the scene outside of a vehicle such as an aircraft or automobile.
2. Description of the Related Art
A HUD is typically any transparent display that presents an image without obstructing the viewer's view or requiring the viewer to look away from the scene outside of a vehicle such as an aircraft or automobile while flying or driving. Initially developed for use in military aircraft, HUDs are used in commercial and private aircraft, automobiles, and other applications. HUD systems may be comprised of a plurality of components including, but not limited to, an image projection unit and a partially transparent and reflective optic commonly referred to as a combiner.
In general, a HUD uses a combination of lenses to create a collimated image. Lens designs that achieve the necessary optical performance tend to be complex and expensive, especially when the field of view (“FOV”) is high and the diameter of the lens is large. For example, FIG. 1 shows an optical layout of a HUD with a wide FOV, which consists of an image projection unit that employs a relay lens assembly mounted over the viewer's head and a combiner unit located forward of the viewer that may be mounted in an aircraft such as a commercial aircraft. As shown, the relay lens assembly is used to form an intermediate image one focal length away from a transparent combiner, which, in turn, functions as a collimating mirror forming collimated beams of light.
In another example, FIG. 2 shows a HUD assembly comprised of an image projection unit and combiner unit that may be mounted in the glareshield of an aircraft such as a military fighter aircraft. As shown, beams of light leave a group of field flattening lenses, strike a reflecting mirror or prism, and pass through convex and concave lenses to form collimated beams of light.
As shown in the examples of FIGS. 1 and 2, the HUD projection units consist of a complex array of lenses, prisms, and/or mirrors. Examples of other complex arrangements are illustrated in U.S. Pat. Nos. 4,178,074 and 4,232,943. It is not uncommon for these HUDs to contain more than 5 lenses, to have diameters greater than 13 cm (5 in), and to weigh more than 13 kg (28.5 lbs). Generally, the size, weight, and complexity of these optical systems detrimentally affect the costs for an operator to employ them.
Attempts to reduce the cost, size, weight, and complexity have been made by others. For example, a reflective HUD system such as that disclosed in U.S. Pat. No. 4,082,432 which “utilizes a single block of transparent acrylic resin with the upper surface curved and coated to form a collimating mirror. The block itself is comprised of two major portions cemented together with a transparent bonding material wherein the resulting interface forms a combining surface.” By its very nature, this block of acrylic is extremely large due to the long eye relief and lack of multiple combiners. Also, a collimating mirror is employed therein, but it is not catadioptric, i.e., does not employ a corrector lens in conjunction with the collimating mirror. If such system were to attempt to employ a corrective lens, such lens would cause a significant distortion of the real world scene because it would have to be placed between viewer and the combining surface of the block. Also, these are large and bulky conventional optics which are heavy and utilize considerable installation space.
Multiple combiners have also been employed. Previous HUD systems may have used multiple combiners to expand the viewer's FOV. In such systems, dual combiners could form two mirror images of a collimating lens, one slightly above and slightly behind the other. Such configuration allows for the use of a smaller collimating lens for the same vertical FOV. In addition, more than two combiners may be used to expand the field. An example of such use is found in U.S. Pat. No. 4,099,841.